The Islamic State, also known as ISIS, posted a video Sunday which it said showed the beheading of U.S. aid worker Peter Kassig, according to various media reports.

Kassig, 26, was captured on October 1, 2013, while en route to eastern Syria. He had founded a nongovernmental organization that provided aid for refugees fleeing the civil war in neighboring Syria. He said he designed his aid organization, Special Emergency Response and Assistance, around the belief that "there was a lot of room for improvement in terms of how humanitarian organizations interact with and cooperate with the populations that they serve."

He had converted to Islam during captivity, according to his parents, taking on the name Abdul-Rahman.

He was apparently not the only captive executed. According to AFP, the militants marched prisoners by a wooden box of long military knives, each taking one as they passed, before forcing their victims to kneel in a line and decapitating them. AFP described the execution as "a highly choreographed sequence."

The executioner in the video appeared to be a militant dubbed "Jihadi John," who is suspected of carrying out beheadings in previous videos released by Islamic State. The man, who is believed to be a British national, was reportedly injured in an air strike mounted by a U.S.-led coalition on a summit of Islamic State leaders in an Iraqi town close to the Syrian border last week.

Kassig appeared last month in an online video that purported to show the same masked militant, who threatened to behead the U.S. Army veteran next, after the apparent beheading of British hostage Alan Henning.

The video and threat were a heartbreaking development for Kassig's family and friends, who have stayed silent since his capture while working to secure his release.

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Kassig's parents issued a statement last month describing their son's work and asking for privacy.

"We ask everyone around the world to pray for the Henning family, for our son, and for the release of all innocent people being held hostage in the Middle East and around the globe," the statement said.

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